This invention is related to flow sensing probes or pitot tubes adapted to sense both a dynamic pressure and a static pressure through a series of openings, and more particularly to a probe having a circular exterior cross-section and rear openings disposed at an angle of about one hundred and ten degrees rearwardly of the forward openings.
Pitot tubes are commonly used for measuring fluid flow. Conventional flow-sensing tubes have a series of forward openings facing toward the direction of flow for measuring dynamic pressure, and a rear series of openings for measuring a lesser or static pressure, to determine flow rates at various flow velocities. Round tubes have the rear openings disposed at an angle of one hundred and eighty degrees with respect to the forward openings. The two pressures are sensed in a pair of internal chambers which in turn communicate with a measuring device.
The problem with conventional flow-sensing tubes is that the fluid pressure varies as it passes around the tube. The pressure differential varies through the range of normal fluid velocities. Two types of calculations are usually required to determine a flow rate. The first calculation is derived from flow tests conducted to determine the actual performance of the device compared to the theoretical. This relationship is expressed as a flow coefficient.
Usually the flow coefficient for prior art devices varies with changes in flow rate. Thus a separate calculation must be made for each flow rate to determine the coefficient for that flow rate.
Attempts have been made in the prior art to provide a more accurate means for sensing fluid pressures in order to compute volumetric flow rate. For example, Lambert U.S. Pat. No. 3,751,982 discloses a pair of rearwardly extending walls carried on the pitot tube to influence the air pressure as it passes around the tube. Other devices of the prior art place the rear openings at a ninety degree angle with respect to the forward openings. Some commercial applications employ a tube having a diamond-shaped cross section to improve the accuracy of the pressure reading.